Capacitors which are subjected to a-c, and particularly capacitors used with fluorescent lamps, frequently use a plastic as a dielectric on which a thin metallized layer is applied to form the electrode. The square or surface resistance of the electrode metallizing coating is approximately 3 ohms when the capacitor is first made. Already at intermediate electrical loading, that is, applying an electrical field across the capacitor, that is, across the dielectric, results in continued deterioration of the metallic coating. When a certain limiting field is reached, the deterioration, which increases with time, is in the form of circular degradation of the metallic layer so that, as the metallic layer degrades, the value of capacity of the capacitor decreases. A typical plastic dielectric is a foil of polypropylene. In such a foil, deterioration has been observed at a field strength of about 40 V/.mu.m, at a frequency of 50 Hz.
Decrease in the capacity value can be avoided by increasing the thickness of the dielectric so that the field strength is reduced. The dielectric, however, can withstand a higher operating voltage from point of view of flash-over or burn-through. For example, fluorescent lamp capacitors using a metallized polypropylene foil or film as a dielectric, and designed for nominal voltages of between 220 to 250 V at 50 Hz, are made with a foil thickness of about 8 .mu.m when using aluminum as a coating; a foil thickness of 6 .mu.m, with a sufficiently thin aluminum coating and a square or surface resistance higher than 3.5 ohms, would be sufficient, however. The consequent field strength with a 6 .mu.m foil would be excessive, however, since the aluminum coating or aluminum electrode surface area would degrade, particularly due to testing which requires 1.25 times the nominal working voltage, or more. The effect of this electrode coating degradation thus increases the cost of the resulting capacitor and also increases its volume more than necessary from a purely electrical insulation point of view.
It has previously been proposed to introduce a uniformly distributed additive of copper to the coating of aluminum in order to prevent degradation of the coating based on applied electrical field. Use of copper is not entirely satisfactory and the results achieved hardly warrant the additional complexity and expense.